Turkey has a large Kurdish population. The northern portion of Iraq is populated predominantly by Kurds. The Turks are afraid of a Kurdistan being established on their borders as part of a post war Iraq.
The Turks have a history of conflict with the Kurds. A long, bloody history.
Turkey says it wants to keep it's border secure by moving troops into northern Iraq. They already have troops inside Iraq, and they've been there for years, but now they want to increase their presence. The latest word is that they want to establish a 20 mile buffer zone along their border.
Their population opposes a war in Iraq, so that is given as the reason for not allowing ground troops, or the use of their airfields.
Some believe the Turks don't want coalition forces between them and the Kurds, and that's part of the reason for initially hindering coalition flights in their airspace after authorizing it's use by those forces.
Turkey also has historic claim to the northern Iraqi oil fields... and most of the Middle East.
It's quite a mess.
